1884.] Notes on a Nevada Shell (Pyrgula nevadensis). 853 
Comparing these formulz, the necessity of complete separation 
rom Amnicola is sufficiently obvious. Whether the produced 
aperture of Pyrgula, as figured by Chenu; is of generic value is 
quite uncertain, for Pyrgula is most certainly a vegetarian. 
Though the genus is said to be confined to the fresh waters of 
the mountainous areas which it inhabits, it is, in the North Amer- - 
ican localities, found in brackish water-lakes alone. 
_ It may profitably be further noted that the dentitions given by 
Troschel, Moquin-Tandon and Stimpson differ very much, even 
more than should be allowed to pass without comment. It has 
been found by Mr. Beecher, and corroborated by the senior 
author, that many descriptions are imperfect and will need revis- 
ion. To this end we invite collectors to send to our addresses 
any of the smaller species of fresh-water univalves which may 
chance to occur in their localities. 
Following is the description of 
THE DENTITION OF PYRGULA NEVADENSIS, 
Jaw thin, membranaceous. 
Odontopore .62™™ in length, and .15™™ in width. There are usually fifty-five 
transverse rows of teeth, arranged according to the formula 3-1-3, which is common 
to the family Rissoidæ, 
Rachis distinct, occupying one-fourth the width of the ribbon. Rhachidian tooth 
(Fig. 1) short and broad, with the infero-lateral angles produced and slightly arcu- 
» On each side of the anterior face is a strong, short, conical 
aa tral lobe truncate. Cusp curved forwards and extended into a strong denticle with 
four smaller ones on each side. The formula of the denticles for this tooth would 
therefore be spate 
Body of intermediate tooth (Fig. 2) subrhomboidal, with the infero-interior angle 
slightly produced and with an angulation in the margin above. From this point 
there is a thickening or ridge extending towards the fixed end. Peduncle longer 
th body of the tooth. Upper margin abruptly curved forwards and marked 
by'seven denticles, of which the third inner one is usually the largest; the formula 
or this tooth may be written 3434-4. 
lateral tooth (Fig. 3) spoon-shaped, with the infero-interior margin angular. 
Upper anterior margin marked with a fringe of about twenty-four denticles decreas- 
l Outer lateral tooth (Fig. 4) falciform, straight along the peduncle. Anterior mee 
gin and extremity denticulated with thirty minute denticles, usually decreasing in 
length towards the distal extremity, but subject to some variation. The denticulate 
margin extends more than one-third the length of the tooth. Peduncle slender and 
nearly straight, 
*Manuel'de Conchyliologi i 
Ga yliologie, Tome 1, 294, Fig. 2029. 
Ellsworth Call, Des Moines, Iowa, or to C. E. Beecher, 273 Hudson ave., 
